This invention relates generally to underwater storage of gas used in offshore drilling and production operations. More particularly, the invention pertains to a method and system for storing gas about and along drilling and/or production risers.
In the drilling and production of offshore wells, it may be advantageous to store gas for use in the drilling and production operations. Large quantities of gas, such as air or nitrogen, have been proposed for example to reduce the weight of drilling fluids being returned in an offshore drilling riser.
A drilling riser is typically used in drilling operations from a floating vessel or platform. The drilling riser extends from above the surface of the body of water downwardly to a wellhead located on the floor of the body of water. The drilling riser serves to guide the drill string into the well and provides a return conduit for circulating drilling fluids (also known as xe2x80x9cdrilling mudxe2x80x9d or simply xe2x80x9cmudxe2x80x9d).
It has been recognized that it is desirable for the drilling fluid pressure in the riser at its lower end (at or near the seafloor) to be approximately equal to the pressure of the surrounding seawater. This effectively eliminates problems that arise from using drilling fluid having a density higher than seawater. One promising way of lowering the effective density of the drilling fluid in the riser is to inject a lift gas at the lower end of the riser. The injected gas intermingles with the drilling fluid and reduces the equivalent density of the column of drilling fluid in the riser to that of seawater.
Drilling a well using a gas lifted drilling riser system requires periodic shutdown of the lift gas injection and de-pressuring of the drilling riser. After completion of the activities that required the de-pressuring, restart of the lift gas injection requires a significant volume of lift gas to re-pressurize and re-establish steady-state, lift-gas flow in the riser.
Air and nitrogen are commonly suggested choices for riser lift gas, with nitrogen preferred for safety reasons. Lift gas for re-pressuring the riser can be supplied by installing a lift gas generator. The size of the lift gas generator can be substantially reduced if lift gas storage is available for storing lift gas produced by the generator when no or little new lift gas is required for the drilling operation.
Storing lift gas in pressurized cylinders on board a typical drilling vessel would require a large number of gas cylinders. The weight and space requirements of onboard gas storage would substantially offset the savings of using smaller sized gas generation equipment. One additional difficulty with onboard gas storage is that lift gas supply pressure varies from maximum storage pressure to atmospheric pressure as gas is withdrawn from storage. Storing lift gas as a liquid would reduce the weight and space requirements for lift gas storage and could eliminate variations in supply pressure. However, liquid storage introduces other concerns related to storage of cryogenic liquid and the logistics of lift-gas resupply. A need therefore exists for an effective storage system for handling gas used in drilling and production operations, such as gas lift operations.
The present invention provides a method and system for storing gas for use in offshore drilling and/or production operations that uses storage chambers positioned along and about a generally upright riser that extends through a body of water. The storage system comprises one or more gas storage chambers positioned along and around an offshore riser and a conduit means operatively connected to the storage chambers for passing gas into and out of the chambers for use in drilling or production operations.